An ordinary adjustable wrench includes the wrench body and an adjustable jaw, a fixed jaw is provided at the front end of the wrench body, the fixed jaw is connected with adjustable jaw to form the size of jaw that can be adjusted, a worm is provided at the bottom of the adjustable jaw to drive the adjustable jaw back and forth, the jaw can be moved back forth by screwing the worm with your thumb. This structure has the following deficiencies in practical application: First, the fingers cannot access screw caps in some special locations of machines, or even if the fingers can access them, the operation locations will obstruct the line of sight, which may cause inconvenience for manual operation and result in improper size of jaw, leading to permanent damage of screws; second, manual operation has limited force and needs space for operation, during the design, problems like excessive interval between the worm and the adjustable jaw may be caused, which may cause the damage of the screw cap corners and make them incapable of being screwed, resulting in losses and troubles; finally, manual operation needs great efforts and has poor effect, which will greatly impact the working efficiency.
Therefore, a lot of electric adjustable wrench have been produced, they adopt electromotor driving as substitute of manual driving to drive the worm to rotate, please refer to Chinese Utility Model Patent: ZL200520074217.3, which disclosed a kind of “Electric Spanner” (Disclosure No.: CN2855649Y), which comprises a jaw and an adjusting nut and is featured by that a reduction electromotor is provided in the main body below said adjusting nut with a said adjusting nut, the gear connected to the rotation axis end of said reduction electromotor is engaged with a gear connected to one end of said adjusting nut, chargeable batteries are provided in the cavity in the handle of said spanner, said batteries are connected with the charging circuit, said batteries are connected to said electromotor through the staggered paralleled double-pole-double-throw button control circuit. This design needs to be improved in the following aspects: first, because the electromotor is vertical to the length of the spanner, the head of the spanner is too large to be used conveniently; second, although the reduction gear set is provided, the electromotor still rotates at high speed, during fast driving, the driving force is insufficient and the driving is not stable; third, during fast driving, the worm may be damaged without a buffering device, which may influence the service life of the electromotor; fourth, too fast speed will result in small force, when screwing the screw cap, it will be flexible. There is a similar patent: CN1868684A, which provided a limit pin, however, the advancing speed of the worm is still high and the head of the spanner is inevitably large.
Please refer to U.S. Pat. No. 6,477,921, which disclosed an electric adjustable wrench, which drives the worm by an electromotor via a belt or chain, however, due to the excessive rotating speed of the electromotor, the problems above still cannot be avoided, similarly, there are U.S. Pat. Nos. 6,966,242 and 4,512,221, because the belts and chains require large space of installation and are easily damaged, the electromotor needs to have high efficiency with a small size, which may cause increase of the cost.
As a further improvement, Chinese utility model patent No.: ZL200520014295.4, which disclosed an electric adjustable wrench (Disclose No.: CN2834793Y), which provides a wrench whose handle is provided with a battery seat for receiving batteries, a DC electromotor is equipped in the front part of said battery seat is provided with, the shaft of said electromotor extends forward and is connected with a rotating shaft with a gear at the end through a bevel gear; said bevel gear is connected encloses the shaft of said electromotor, a further improvement, said bevel gear is provided with a supporting spring at the rear part to push said rotating shaft; a contact cut-off switch serially connected with the power circuit is provided beside said supporting spring is provided. In this patent, said electromotor is provided inside the spanner wrench and parallel to the length of the wrench body and is driven by the bevel gear, which make the head of the wrench body compact (U.S. Pat. No. 7,114,824 provides a similar structure); the supporting spring works as the buffering device, which reduces the abrasion of internal mechanical structures. Though great improvement has been made, the advancing speed of the worm is still too high, as the output power of said electromotor is rated, the force is still insufficient, and when screwing the screw cap, the worm is still subject to looseness; the electromotor produces large noise during the driving, and further improvements can be made.